Assessment of Three-Dimensional Trunk Kinematics and Muscle Activation during Cycling with Independent Cranks.

Cycling has been exposed to technological advances that claim to enhance performance. Normally, with regular bicycle cranks, cyclists push down on one pedal with more force and velocity than the contralateral leg can pull up. In effect, some of the downward leg's force is used to push up the contralateral recovering leg, causing a resistance force (Bini et al., 2013). The introduction of clipless pedals, which securely connect the shoe to the pedal, allow cyclists to apply a pulling force on the leg responsible for the resistance, thereby increasing cycling effectiveness (Mornieux et al., 2008). Independent cranks (IC) are recently introduced bicycle cranks that use a one-way clutch design to decouple the cranks, allowing each leg to pedal independently of one another.

The full pedal cycle can be separated into two phases, the push phase and the recovery (pull) phase. Cycling with IC requires active flexion of the hip and knee, as well as dorsiflexion of the ankle for stabilization in the recovery phase. This is necessary for cyclists to maintain an anti-phase cycling pattern, where the cranks remain 180[degrees] apart. IC manufacturers have claimed that the active flexion of the hip and knee facilitates training of specific muscles, which results in decreased resistance to the contralateral leg in the push phase when cycling with normal cranks (NC). Previous research on IC has primarily observed physiological factors by studying changes in power output (Bohm et al., 2008), gross efficiency (Burns et al., 2012; Luttrell and Potteiger, 2003) and oxygen uptake (Burns et al., 2012). Improvements to these physiological variables were absent in all of the aforementioned studies except for Luttrell and Potteiger (2003), who found decreased energy expenditure during a 1-hour submaximal ride after 6 weeks of IC training. However, few studies have observed IC cycling from a biomechanical perspective. Burns et al. (2012) observed muscle recruitment patterns of the vastus lateralis, biceps femoris and gastrocnemius after a five week training period with IC, and found no difference in recruitment patterns upon returning to NC cycling (Burns et al., 2012). Their study only collected sEMG of three left leg muscles, and did not include a hip flexor muscle, which is important to observe due to the active flexion requirement during IC cycling. Muscle activation during submaximal IC cycling was also studied by Hug et al. (2013) for 10 muscles of the left leg. Their study considered hip flexor muscles and it was found that during cycling at a 100 watt work rate, there was a significant increase in muscle activity of the tibialis anterior, gastrocnemius medialis, rectus femoris, biceps femoris, semimembranosus, and tensor fascia latae (Hug et al., 2013) when cycling with IC. Their study, although considering hip and knee flexor activation, only observed muscles unilaterally. Due to the independent nature of the cranks, it is important to observe muscles bilaterally in order to consider any asymmetry between legs; a concern for injury risk due to uneven force distribution (Smak et al., 1999). It is also important to observe muscle recruitment between legs at an increased work rate; a 100 watt load is very light and increased muscle fiber recruitment may be seen at higher loads that are more representative of a work rate experienced in training.

To the best of the authors' knowledge, there has been limited analysis of cycling kinematics when cycling with IC. Previously, cycling kinematics with NC during fatigue was studied, and it was found that the trunk demonstrated increased anterior flexion at the end of an exhaustive cycling test (Dingwell et al., 2010; Sayers and Tweddle, 2012). Conversely, it was found that there was no effect of workload on trunk angles when fatigue was not a factor (Bini et al., 2016). These findings suggest that trunk kinematics are only altered when there is a fatigue effect, or workload and fatigue effects are combined. The aforementioned studies have observed three-dimensional kinematics with NC. However, no studies have compared three-dimensional trunk kinematics between NC and IC cycling. It is important to evaluate the trunk kinematics of IC cycling, as this may have implications with lumbar spine loading while training. Due to cyclists being in a seated position, there is a constant flexion of the lumbar spine, putting them at risk for low back pain (Callaghan and Jarvis, 1996; Manninen and Kallinen, 1996). Additionally, activities that involve repetitive flexion/rotation are associated with flexion pattern pain disorder (O'Sullivan, 2000). Therefore, increased trunk rotation, lateral flexion, forward flexion, and more importantly, overall range of motion (ROM) in the sagittal plane could increase the risk for low back pain in cyclists.

The goal of this study was to gain a greater understanding of the three-dimensional kinematics of the lumbar spine and bilateral muscle recruitment patterns during a graded exercise test during both IC and NC cycling. It was hypothesized that there would be a greater increase in trunk ROM around all three axes during IC cycling. It was also hypothesized that due to reduced assistance from the contralateral leg in the recovery phase, there would be increased muscle activation in the rectus femoris and biceps femoris during IC cycling compared to NC cycling as external load is increased.

Methods

Experimental approach

Kinematics of the trunk and sEMG of six muscles bilaterally were used to evaluate differences between IC and NC cycling during a graded exercise test.

Participants

Ten healthy, physically active male university students were recruited to participate in this study (Table 1), which was approved by the Nipissing University Research Ethics Board (REB: 100669). All participants provided written and informed consent prior to any data collection. The participants were considered recreational cyclists; any previous cycling experience was for leisure purposes (not competition). As IC have been advertised as a training tool for many sports and rehabilitation purposes, elite cyclists were not included in the participant pool to assess the effect on healthy, recreational cyclists. Participants with any...